A conventional road bicycle ("road bike") is generally equipped with two chainrings on the crank and a freewheel containing five to eight gears or sprockets. A road bike freewheel includes sprockets which, in some instances, are only one gear tooth apart from each other, producing a relatively narrow range of gear ratios. A road bike is, in general, configured to be ridden at relatively high speeds on smooth surfaces.
Road bikes are, in many instances, equipped with shifting levers having relatively small take-up spool diameters that produce a relatively small control cable displacement per angular displacement of the spool. This relatively small cable displacement is used to laterally shift a rear derailleur from one sprocket to the next. Road bicycle derailleurs thus have a relatively high actuation ratio, generally defined as the amount of derailleur movement perpendicular to the planes of the freewheel sprockets per unit displacement of the derailleur control cable.
More recently, mountain bicycles ("mountain bikes") have been developed which are ridden on trails that are not at all smooth; the "technical" portions of these trails commonly include sharp inclines, large boulders and tree trunks. The mountain bike freewheel thus includes a sprocket set that has a wide range of gear ratios. Further, mountain bikes are commonly exposed to very dirty and muddy conditions, necessitating improvements in keeping the mechanical components, particularly the derailleur system, of the mountain bicycle from becoming contaminated.
As illustrated in FIG. 1, the conventional rear derailleur takes a parallelogram form, defined by points (i.e., pivots) A, B, C and D. A b-knuckle 200 (also referred to as a b-pivot), is affixed to the bicycle frame 202 (see FIG. 2) by conventional means, such as a bolt 204 and a derailleur hanger portion 206 of the bicycle frame 202. Inboard and outboard sideplates 208, 210 hingedly connect this b-knuckle 200 to a p-knuckle or p-pivot 212, which in turn has rotatably affixed to it an idler cage 214 containing two guide wheels 216, 218 for guiding the drive chain 220 onto the selected sprocket of the freewheel 222.
As illustrated in FIG. 2, the conventional derailleur system is generally actuated by a cable system 226. The cable system 226 includes a derailleur control cable 228 which is commonly a Bowden type-that is, the cable 228 is contained within a sheath 230 that terminates in a ferrule 232 affixed to the b-knuckle 200. The cable 228 continues to a clamping screw 234 or the like that clamps the cable end to one of the sideplates 208, 210, such as the outboard sideplate 210. As the cable 228 exits the ferrule 232, the cable 228 is directed in a first direction. The cable 228 is also commonly clamped to the sideplate 210 in a second direction, and this can often be quite different from the first direction depending on how far inboard or outboard the p-knuckle 212 has been pulled by the cable 228.
In many conventional derailleur systems, the cable 228 makes a sharp angle to the ferrule 232 when the p-knuckle 212 is in an outboardmost position. This produces excessive friction and, hence, wear on the cable 228 at the points where the cable 228 rubs against the ferrule 232. In addition to the excessive wear, the shift in position of the p-knuckle 212 causes the actuation ratio to change as a function of the sprocket to which the upper guide wheel 216 of the idler cage 214 is aligned.
Mountain bike designs have, of recent, employed a hand-rotatable shift actuator coaxial to, and bearing directly upon, the handlebar to displace the derailleur control cable. Illustrative is the hand-rotatable shift actuator disclosed in U.S. Pat. No. 5,134,897.
The amount of displacement of the control cable ("cable pull") in the noted patents is dramatically increased in comparison to conventional shift levers mounted on the frame by virtue of the relatively large radius of the handlebar. Certain hand-rotatable shift actuator manufacturers have attempted to minimize the cable pull by reducing the cable spooling radius (i.e., radius of handlebar plus spool thickness) and eliminating a protective sleeve or mandrel, such as that disclosed in U.S. Pat. No. 5,134,897. This is an unsatisfactory solution given the increased wear and, hence, reduced structural integrity of the handlebar.
In order to reduce the actuation ratio, the noted manufacturers have also lengthened the cable clamp to pivot distance in the derailleur. However, due to the inherent limitations of conventional derailleur designs, this modification does not achieve sufficient reduction in the actuation ratio to employ a protective sleeve or mandrel on the handlebar while maintaining optimum performance characteristics (e.g., sufficient mechanical advantage over the derailleur, sufficient degrees of rotation per shift). A need therefore exists among bicycle manufacturers for a rear derailleur system having a substantially uniform and low actuation ratio while permitting the use of a simple protective spool or mandrel coaxially mounted on a handlebar.
Conventional derailleurs have an additional drawback in that the idler cage (which is used to keep the chain properly engaged to the guide wheels) is used to push the drive chain inboard onto the larger sprockets of the freewheel. Since the angular position of the idler cage in conventional derailleur systems changes according to which sprocket the guide wheel is aligned with, the idler cage will often inhibit the lateral flexing of the drive chain which is necessary for easy shifting from one sprocket (i.e., gear) to another. Instead, the idler cage in the noted conventional systems laterally pushes the drive chain over to the larger sprocket. Mechanically, this displacement of the drive chain by the idler cage by brute force produces a rough, abrasive shift that requires a relatively large amount of force. As will be recognized by those skilled in the art, this problem is exacerbated during rapid multiple shifting.
Some conventional derailleur manufacturers have attempted to optimize lateral flexing of the drive chain by employing various modifications (e.g., cage geometry) to the idler cage. Such modifications have been found unsatisfactory due to the idler cage rotation (resulting from shifting) which adversely alters the relationship between the idler cage and the drive chain.
As discussed above, a conventional rear derailleur control cable is generally housed within segments of a cable housing along certain portions of its length. As illustrated in FIG. 2, the cable housing segment 250 proximate the rear derailleur conventionally extends in a 180.degree. reverse loop from the chainstay 252 to a ferrule 232 on the b-knuckle 200 of the rear derailleur. When used on mountain bikes, it has been found that this Bowden cable housing length collects water, mud, grit and the like. The foreign matter will have a tendency to collect in the Bowden cable housing, resulting in jamming or seizing of the cable and, hence, increasing the force required to actuate the derailleur. The cable will also have a tendency to rust at this point because of its exposure to water. A need therefore exists for a control cable design which obviates the collection of water and foreign matter within the cable housing.
An additional objective of a good derailleur/freewheel design is to have a relatively constant freewheel gap between the upper guide wheel and each of the sprockets, regardless of which sprocket the upper guide wheel is presently aligned with. This has been conventionally met by such expedients as displacing the upper guide wheel axis from the p-knuckle pivot and b-knuckle pivot. However, each additional degree of freedom built into the rear derailleur provides another chain resonating frequency when the bicycle goes over a series of bumps. This resonance will cause the chain to bounce, disengaging from the chainrings in extreme instances. Further, an offset of the guide wheel from the p-pivot creates problems in optimization of the chain gap; while a relatively constant chain gap can be specified for one particular front chainring, a shift to another chainring means that the chain gap is no longer optimized. These problems have created a need for a rear derailleur/freewheel design that is more conducive to the sport of mountain cycling.